Bony - Wrong calculation in `PerpMath._profitLoss`

[sherlock-admin]

Bony

medium

Wrong calculation in PerpMath._profitLoss

Summary

PerpMath._profitLoss function doesn't calculate the PnL correctly

Vulnerability Detail

    function _profitLoss(FlatcoinStructs.Position memory position, uint256 price) internal pure returns (int256 pnl) {
        int256 priceShift = int256(price) - int256(position.lastPrice);
        int256 profitLossTimesTen = (int256(position.additionalSize) * (priceShift) * 10) / int256(price);

        if (profitLossTimesTen % 10 != 0) {
            return profitLossTimesTen / 10 - 1;
        } else {
            return profitLossTimesTen / 10;
        }
    }

We can think of the following condition A.

• priceShift is 5
• position.additionalSize is 42
• price is 50 In this case, the profitLossTimesTen is 42 * 5 * 10 / 50 = 42

We can also think of the following condition B.

• priceShift is 5
• position.additionalSize is 40
• price is 50 In this case, the profitLossTimesTen is 40 * 5 * 10 / 50 = 40 In condition A the PnL is 3 and condition B, PnL is 4. It is unfair for users.

Impact

Incorrect calculation of _profitLoss will lead to user fund loss

Code Snippet

https://github.com/sherlock-audit/2023-12-flatmoney/blob/bba4f077a64f43fbd565f8983388d0e985cb85db/flatcoin-v1/src/libraries/PerpMath.sol#L175

Tool used

Manual Review

Recommendation

    function _profitLoss(FlatcoinStructs.Position memory position, uint256 price) internal pure returns (int256 pnl) {
        int256 priceShift = int256(price) - int256(position.lastPrice);
        int256 profitLossTimesTen = (int256(position.additionalSize) * (priceShift) * 10) / int256(price);

-       if (profitLossTimesTen % 10 != 0) {
-           return profitLossTimesTen / 10 - 1;
-       } else {
-           return profitLossTimesTen / 10;
-       }
+       return profitLossTimesTen / 10;
    }

Duplicate of #286

[sherlock-admin]

1 comment(s) were left on this issue during the judging contest.

takarez commented:

valid: function is not being accurate as of current implemenation; medium(8)